This invention relates generally to wind turbines, and more particularly to methods and apparatus for increasing energy capture and for controlling twist angles of blades resulting from passive twist bend coupling.
Recently, wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted within a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid.
Studies have shown benefits of passive pitch control of rotor blades (i.e., twist-bend coupling, or TBC) to attenuate extreme blade loads. Within a twist-bend coupled section of a rotor blade that is made of laminate material, the laminates of the blade undergo shear fatigue resulting from continuous passive pitching. This fatigue presents a risk to TBC technologies. However, very little, if any, research has been devoted to the study of how laminates used in the manufacturing of rotor blades respond to shear axis fatigue.
At least one known wind turbine utilizes torsionally stiff blades without twist-bend coupled blades. Only a single tip speed ratio (i.e., rotation rate divided by wind speed) is tracked and used to maintain a maximum power coefficient. More specifically, rotor speed rises with wind speed in such a manner as to maintain an optimized or nearly optimized tip speed ratio over a certain period of time. Tip speed ratio is known in the industry and is generally a limiting factor in blade rotational design. This limit results from aerodynamic noise.
At least one known wind turbine uses twist-bend-coupled blades that passively pitch to feather a relatively small amount when loaded by aerodynamic loads. Particularly in response to strong wind gusts, this passive pitch tends to balance out asymmetric loads across the rotor disk and reduces system fatigue damage. However, due to passive pitching of the TBC, energy capture below rated wind speed is reduced slightly compared to a non-coupled blade with an identical aerodynamic envelope. More specifically, optimum pitch setting for maximum energy capture varies with wind speed. To partially compensate for the loss of energy capture, at least one known configuration provides a pre-twist (i.e., a twist bias built into the rotor blades), but this bias is most effective at only one wind speed. Energy loss still occurs at other wind speeds. However, pre-twist in the TBC section mitigates power loss at below-power rated wind speeds.
For large-scale wind turbines, rotor blades require pitch actuation at the blade root to actively adjust the pitch angle or angle-of-attack of the rotor blade. However, during operation, fine pitching of the outboard section of the rotor blade can be achieved through passive pitching by means of a Twist-Bend Coupling (TBC). Passive pitching of a several degrees is achieved in the TBC by means of blade construction and specifically due to laminates orientation and lay-up. For example, in the TBC section the fiber matt material is orientated to allow the blade to passively pitch under specific loading conditions. This pitching reduces aerodynamic lift by passively pitching slightly towards a feathered position and therefore reduces blade loading. TBC is a feature that has been shown to reduce the demand on the active pitch mechanism, which is typically located at the root of the rotor blade. Thus passive TBC can reduce pitching power requirements and associated fatigue and damage to the active pitch axis system, located near the blade root.